The present invention relates to a photelectric display apparatus, and more particularly to a luminous faceplate of the color projection cathode ray tube (CRT) and the substrate for manufacturing the same.
One of the apparatus for displaying pictures and words on large screen is color projection television. The most critical parts of the projection apparatus are three projection CRT""s, which projects red, green and blue light, respectively. Usually, this kind of the luminous faceplate of projection CRT is formed by depositing powder phosphors on a glass substrate. The glass substrate has a very low thermal conductivity, and the heat conducting performance of the luminous powder is also not good, resulting in that the glass-powder luminous faceplate could not work at relatively high operating power. Furthermore, the luminous powder will be decomposed at a power density higher than 1W/cm2; and the luminous powder will be melted under the power density of 5W/cm2. On the other hand, increasing area of the electron-beam spot in order to decrease the power density would adversely impair the resolution of a CRT. Besides, there is so-called Coulomb Degradation existing in the luminous powder: when the electron dosage reaches 150 coulombs/ cm2, the initial intensity of the luminous powder will decrease to a value of 50% of the initial intensity. Even in the preferred operation environment, the lifetime of this kind of CRT""s under high luminance is only 1000 hours.
In order to overcome the above-mentioned weaknesses of the glass-powder luminous faceplate, a new kind of the color projection luminous faceplate has been developed. This kind of luminous faceplate is composed by epitaxially growing a rare-earth element doped yttrium aluminum garnet (YAG) luminous layer having a high thermal conductivity on a YAG single crystal substrate, which has very high thermal conductivity. Because of the good thermal contact between an epitaxial luminous layer and a heat-conductive substrate, this kind of luminous faceplate can work under a high operating power which would destroy the glass-powder luminous layer. Thus, the color projection CRT""s made of this kind of epitaxial luminous faceplate having highly thermal conductive can operate under a high power, and has very high resolution and very short relaxation time. It further has the advantages of corrosion resistance and aging-resistance.
However, it is very difficult to bond a YAG single crystal wafer and a CRT body together because of the relatively high difference between their thermal expansion coefficients. YAG single crystal has a refractive index of 1.84, the corresponding light output fraction from the epitaxial faceplate in only 16%, thus the brightness is not high enough. These two factors result in that the projection CRT made of epitaxial luminous faceplate on the YAG single crystal can not be practically used.
Hereafter, Chinese patent No. 95111324.0 discloses a technique for solving the problem for hermetically bonding a substrate of single crystal YAG to a tube body, and increasing the brightness of the single crystal YAG luminous faceplate. By means of this technique, a color projection CRTs having a YAG single crystal luminous faceplate with high definition and high brightness was successfully fabricated, the resolution of which reached 2000 lines and brightness of which reached 105 cd/m2.
However, the growth rate of the YAG single crystal used as the substrate of the luminous faceplate is low, while the high temperature puller for preparing the crystals is very expensive and consumes huge electrical power. Thus, using YAG single crystal as the substrate of luminous faceplate is unfavorable for mass production of the luminous faceplate and for reducing of cost. In addition, since size of the faceplate is limited by the size of the single crystal ingot (at present, it can only be made to 4 inches), using YAG single crystal as the substrate of luminous faceplate is therefore unfavorable for increasing the operation power of the CRTs. It is therefore needed to find a material for luminous faceplate of color projection CRTs, which can satisfy with requirements for performance specifications of the luminous faceplate and be mass-produced at low cost as well.
In the prior art, transparent polycrystal body (TPB) for various transmission windows has been investigating for many years. The technology for producing TPB is relatively easy, throughput is high, production cost is relatively low, and particularly some TPBs have very high mechanical strength and excellent corrosion-resistance. Therefore, it is widely used in many especial fields such as sensing tip cover of the missile, monitor window of high temperature furnace, lens for detecting radiation in a well, jet nuzzle, and so on. The inventors of the present invention tried to use TPB in the photo-displaying field, and found that once the TPB satisfies with certain requirements on physical and chemical characteristics such as transmittance, thermal conductivity, mechanical strength, and corrosion-resistance, it can be used to fabricate the luminous faceplate of CRTs.
According to the mature technology for fabricating TPB, TPB ingot is formed by means of one-step vacuum hot press sintering or two-steps vacuum sintering, and thermal isostatic pressing. The ingot is cut into desired shape and size, then is ground and polished to desired smoothness, so that the substrate of TPB is formed. A layer of luminous film is formed on one of the substrate surfaces, whereby luminous faceplate suitable to be used in projection CRTs is formed.
The size limit of the substrate of TPBs mainly determined by the size of the furnace chamber of hot press or a hot isostatic press. There is no problem in fabrication of TPB having a diameter larger than 200 mm and a thickness larger than 5 mm. The size of the substrate of TPB may be much larger than that of the single crystal substrate, its processing is also much faster.
The substrate is usually in plane circle shape. The inventors of the present invention further found when a large screen luminous faceplate used under high power is fabricated, it has several advantages to use a substrate of TPB in the shape of thickness-equal spherical face. That is, it can not only increase the mechanical strength of the faceplate, and make the luminous intensity of the faceplate more uniform, but also increase the light output of the epitaxial luminous faceplate.
The object of the present invention is to provide a luminous faceplate of color projection CRTs capable of operating under high power, having low fabricating cost, and easy to be mass-produced.
In order to achieve the object of the present invention, a luminous faceplate used in color projection CRTs is provided, which includes a substrate and luminous layer connected to said substrate, wherein said substrate is consisted of TPB.
Using TPB as material of a luminous faceplate of the color projection CRT has many advantages compared with using a single crystal substrate. For instance, the size of a transparent luminous faceplate can be made much larger than available largest size of the single crystal luminous faceplate; the mechanical strength of TPB is higher than that of the single crystal; TPB is isotropical and easy to be hermetically soldered with CRT body; the production cost (especially in case of forming luminous faceplate in a non-plane) is relatively low, and the produce period is shorter than that of single crystal preparation. Therefore, TPB is suitable for mass production. Since YAG single crystal substrate is formed by cutting single crystal ingot, it is very difficult to make the single crystal substrate to be curve shape, and the material will be seriously wasted. For TPB, however, the only thing has to do is changing the molder from plane to corresponding curve shape. This is a further advantage of TPB superior to the single crystal material in the case of forming luminous faceplate of high power CRTs.
Many kinds of TPBs have extremely high thermal conductivity. Therefore, compared with luminous faceplate of a glass substrate, the luminous faceplate formed by using TPB as the substrate can work at much higher operating power. With the same requirements of brightness and resolution, the color projection CRT formed by using a TPB luminous faceplate of 3 inches diameter can have the same effect as the color projection CRT formed by using a glass luminous faceplate of 7 inches diameter does. Owing to the reduction of the diameter of the luminous faceplate, the size of the overall projection lens is also reduced. This is not only able to reduce the volume of the projection television, but also can reduce the cost. Just because this, although the price of TPB is higher than that of glass, the cost of projection head (projection CRT plus projection lens) is lower than that of the projection head formed by glass faceplate. In addition, since the mechanical strength of TPB is higher than that of glass, and the corrosion-resistance of it is high, the thickness of the substrate can be made thinner than that of glass for fabricating the luminous faceplate having the same size. This can not only reduce the absorption of incident light, but also reduce the image distortion resultant from the thickness, whereby increase the resolution.